The Neutron star Interior Composition Explorer (NICER) is an International
Space Station (ISS) payload devoted to the study of neutron stars through soft
X-ray timing. Neutron stars are unique environments in which all four
fundamental forces of nature are simultaneously important. They squeeze more
than 1.4 solar masses into a city-size volume, giving rise to the highest
stable densities known anywhere. The nature of matter under these conditions
is a decades-old unsolved problem, one most directly addressed with
measurements of the masses and, especially, radii of neutron stars to high
precision (i.e., better than 10 percent uncertainty). With few such
constraints forthcoming from observations, theory has advanced a host of
models to describe the physics governing neutron star interiors; these models
can be tested with astrophysical observations.

NICER will enable rotation-resolved spectroscopy of the thermal and
non-thermal emissions of neutron stars in the soft (0.2-12 keV) X-ray band
with unprecedented sensitivity, probing interior structure, the origins of
dynamic phenomena, and the mechanisms that underlie the most powerful cosmic
particle accelerators known. The NICER mission achieves these goals by
deploying an X-ray timing and spectroscopy instrument on the International
Space Station (ISS).

By answering a long-standing astrophysics question - How big is a neutron
star? - NICER will confront nuclear physics theory with unique measurements,
exploring the exotic states of matter within neutron stars through
rotation-resolved X-ray spectroscopy. The capabilities that NICER brings to
this investigation are unique: simultaneous fast timing and spectroscopy, with
low background and high throughput. NICER will also provide continuity in
X-ray-timing astrophysics more broadly, post-Rossi X-ray Timing Explorer,
through a Guest Observer program. Finally, in addition to its science goals,
NICER will enable the first space demonstration of pulsar-based navigation of
spacecraft, through the Station Explorer for X-ray Timing and Navigation
Technology (SEXTANT) enhancement to the mission, funded by the NASA Space
Technology Mission Directorate's Game-Changing Development program.

NICER's X-ray Timing Instrument (XTI) represents an innovative configuration
of high-heritage components. The heart of the instrument is an aligned
collection of 56 X-ray "concentrator" optics (XRC) and silicon drift detector
(SDD) pairs. Each XRC collects X-rays over a large geometric area from a
roughly 30 arcmin2 region of the sky and focuses them onto a small
SDD. The SDD
detects individual photons, recording their energies with good (few percent)
spectral resolution and their detection times to an unprecedented 100
nanoseconds RMS relative to Universal Time. Together, this assemblage provides
a high signal-to-noise-ratio photon-counting capability within the 0.2-12 keV
X-ray band, perfectly matched to the typical spectra of neutron stars as well
as a broad collection of other astrophysical sources.

From NICER's ISS platform, a star-tracker-based pointing system allows the
XTI to point to and track celestial targets over nearly a full hemisphere.
The pointing system design accommodates the ISS vibration and contamination
environments, and enables (together with NICER's GPS-based absolute timing)
high-precision pulsar light-curve measurements through ultra-deep exposures
spanning the 18-month mission lifetime.

Simulated NICER count rates and spectra can be derived using the WebPIMMS and WebSPEC tools.
The Viewing tool can be used to determine the times when a specific sky
position is potentially visible to NICER.

More details are availbale in NICER's
Mission Guide. A 12-slide overview of NICER science is available
here.

NICER sees the contraction of the corona in a black hole X-ray transient(14 Jan 2019)
NICER made the cover of Nature with an analysis by Kara et al. of the temporal behavior of the X-ray spectrum of the black hole transient system MAXI J1820+070. Their analysis of the time-resolved NICER spectra of this system showed that the inner edge of the accretion disk is near the accreting black hole and that variations in the X-ray spectral lags indicated that the corona shrank over a time interval of a few weeks.

NICERDAS Version 5 released(24 Oct 2018)
The NICER data reduction software Version 5 was released on Oct 23 2018 with the release of
HEASoft 6.25 . NICERDAS 5.0 includes updates to most tasks and includes new task, nicertimeconv, to do basic time conversions.

NICER Guest Observer Program approved by NASA(07 Sep 2018)
NASA has approved a Guest Observer program for NICER. We expect that the Announcement of Opportunity will be released this fall, with proposals due by the end of the calendar year. More information will be posted on the NICER home page when available.

NICER and NuSTAR Identification of X-ray Transient IGR J17591-2342(17 Aug 2018)
Observations by NICER and NuSTAR of the X-ray transient IGR J17591-2342 discovered by INTEGRAL on August 12 show that the new transient is an accreting millisecond pulsar in outburst. The pulsar has a spin frequency of approximately 527 Hz and an orbital period of 0.37 days.